1. Technical Field of the Invention
The present invention relates to a fluid-pressure transmitting apparatus for working vehicles such as wheel loaders and bulldozers and, more particularly, to a fluid-pressure transmitting apparatus having a pilot hydraulic circuit for a capacity control device of a traveling pump.
2. Description of the Related Arts
Conventionally, the working vehicle in various kinds, including wheel loaders and bulldozers, has a pump to be driven by a drive-source engine and a pilot operation valve for varying the delivery capacity from a traveling pump or selectively supplying the delivery flow rate of a working pump to various actuators such as cylinders. The delivery capacity of pump varies depending upon an operation amount of a pilot operating lever. In the case that the pump is desirably controlled for reducing the pump delivery capacity for the purpose of engine-stall prevention or vehicle brake rather than the pump delivery capacity as determined by the control amount of the pilot operating lever, reduced is the pressure of the pressurized-oil to be supplied to the pilot operation valve of the pilot operating lever.
FIG. 5 shows one example of a hydraulic circuit of a related-art fluid-pressure transmitting apparatus having a brake pedal. In the figure, a fluid-pressure transmitting apparatus 1 has a fixed-capacity working-machine pump 2 to be driven by a not shown engine, a fixed-capacity pilot pump 3 and variable-capacity traveling pumps 4 in the left and right. The traveling pump 4 has a pump-capacity control device 5. The pump-capacity control device 5 varies the delivery capacity of the traveling pump 4 by a pilot oil pressure dependent upon an operation amount of the pilot operating lever 6a of the traveling operating device 6. The working-machine pump 2 is connected to various actuators such as arm cylinders and bucket cylinders, not shown, through operation valves 15, 16. The traveling pumps 4 are connected to a not-shown traveling motor.
The pilot pump 3, a hydraulic source for supplying pressurized oil to the traveling pilot hydraulic circuit and the working-machine pilot hydraulic circuit, has a delivery passage 3a branched with an oil passage 3b, 3d connected to the traveling operating device 6 and an oil passage 3c connected to the working-machine operating device 7. The pressurized oil outputted from the pilot pump 3 is supplied at nearly a constant pressure by the relief valve 8 to the branched two oil passages 3d, 3b and to the oil passage 3c. A fixed restriction 9 is inserted on the traveling oil passage 3d while a switching valve 10 is connected downstream the fixed restriction 9, to close the passage of from the oil passage 3d to the oil passage 3b and communicate the oil passage 3b with a tank 11 depending upon a depression amount of a pedal 10a. 
The switching valve 10 outputs a pressurized oil depending upon a depression amount of the pedal 10a. In the supply position A of the switching valve 10 shown in FIG. 5, the switching valve 10 is in a full open state to directly output the pressurized oil from the pilot pump 3. When depressing the pedal 10a down, the switching valve 10 switches over to a close position B opposite to the supply position A. In the close position B, the oil passage 3b connected to the traveling operating device 6 communicates with the oil tank 11 to return the pressurized oil of the oil passage 3b to the oil tank 11.
The traveling and working-machine operating devices 6, 7 each has a pilot operating lever 6a, 7a shown in FIG. 6 and pilot operating valves 6b-6e, 7b-7e shown in FIG. 5. Each pilot operating valves 6b-6e, 7b-7e, as pressure-reducing valves, output a pilot pressure depending upon an inclination amount (operating amount) of the operating lever 6a, 7a. 
As shown in FIG. 5, the output pressure of the traveling pilot operating valve 6b, 6c is selectively outputted to a forward or backward pilot pressure-receiving part in the pump capacity control device 5 of the traveling pump 4 through a pilot-pressure admission passage 13a or 13b connected to the bridge circuit 12, by forwardly or backwardly operating the pilot operating lever 6a. 
The working-machine pilot operating valves 7b-7e are respectively connected, for example, to an arm valve 15 and a bucket valve 16 through pilot-pressure admission passages 14a-14d. The output pressure of the arm pilot operating valve 7b, 7c is selectively outputted to a pilot pressure-receiving part of the arm valve 15 through the pilot-pressure admission passage 14a, 14b by operating the pilot operating lever 7a in either upper or lower direction. The output pressure of the bucket pilot operating valve 7d, 7e is selectively outputted to a pilot pressure-receiving part of the bucket valve 16 through the pilot-pressure admission passage 14c, 14d by operating the pilot operating lever 7a in either direction toward digging or dumping.
When the traveling operating lever 6a is operated in a desired direction, e.g. when the operating lever 6a is inclined toward the forward shown in FIG. 6, outputted is a pressure through the corresponding pilot operating valve 6b depending upon an inclination amount of the pilot operating lever 6a. At this time, the output pressure of the other pilot operating valves 6c-6e remains a pressure in the tank 11. In the case that the pump capacity of the traveling pump 4 is desirably reduced lower than a pump capacity as determined by a current operation amount of the pilot operating lever 6a for the purpose of brake or engine-stall prevention, the pedal 10a is depressed down.
When depressing the pedal 10a, the switching valve 10 is switched from the supply position A toward the close position B. Because the delivery pressurized oil of the pilot pump 3 supplied to the pilot operating valve 6b is reduced in pressure to decrease the pressure of the pressurized oil outputted from the pilot operating valve 6b, the pump capacity of the traveling pump 4 can be decreased lower than a pump capacity as determined by an operation amount of the pilot operating lever 6a. This makes it possible to reduce the speed of the vehicle lower than a speed as determined by an operation amount of the pilot operating lever 6a or stop the same.
Meanwhile, there are disclosed examples of hydraulic circuits for varying the pump delivery capacity depending upon an engine rotational speed, e.g. in Japanese Patent Laid-Open Nos. 71353/1974 and 122363/1998. In the former, the pump capacity is increased with the increase in engine speed, wherein, at low rotational speed of the engine, the pilot oil pressure is cut off to decrease the pump capacity. In the latter, when the engine rotational speed increases exceeding a predetermined level, the pressure of a pilot oil-pressure source is decreased to reduce the capacity of a variable capacity pump.
In the related-art fluid-pressure transmitting apparatus exemplified in FIG. 5, in the traveling pilot hydraulic circuit the switching valve operated by the pedal is arranged on the output side of the pilot pump so that the delivery pressurized oil of the pilot pump is supplied to the traveling operating device having the pilot operating lever through the switching valve. The switching valve is usually in a communication state, and reduced in opening degree depending on a depression amount thereof to gradually decrease its output to the traveling pilot hydraulic circuit.
In the meanwhile, the arrangement of the working-machine pilot operating lever and traveling operating lever is different in various forms depending on its vehicle kind and manufacturer. For this reason, in order to secure the common operationality to the operating levers for various working vehicles, there has been an attempt to switch such that, for example, the working-machine pilot operating lever at its output is connected to the capacity control device of the traveling device and that the traveling pilot operating lever at its output is connected to the operation valve of a working-machine actuator, thereby modifying the operation pattern in the plot operating lever, for example, to allow an operator accustomed for other manufacturer s vehicle to drive his company-make vehicle in the same operating pattern.
In the related-art fluid-pressure transmitting apparatus of FIG. 5, there is the switching valve to reduce the pressure of the pressurized oil from the pilot pump between the input side of the traveling pilot operating lever and the pilot pump, but the input of the working-machine pilot operating lever connects directly to the output of the pilot pump. Also, the traveling pilot operating lever at its output is connected to the capacity control device of the traveling pump while the work equipment pilot operating lever at its output is connected to the work equipment operating valve.
Namely, the pilot hydraulic circuits on the traveling and work equipment sides constitute independent, different circuits from each other. For this reason, it is not satisfactory to merely switch, for connection, the piping on the output side of the working-machine pilot operating lever and the piping on the output side of the traveling pilot operating lever. There is a need to simultaneously switch the connection over between the piping for supplying pressurizing oil to the work equipment pilot operating lever and the piping for pressurized oil supply to the traveling pilot operating lever.
In order to realize such connection, the arrangement of piping must be modified on the input and output sides of the work equipment and traveling pilot operating levers, or otherwise a two-stage switching valve or the like be provided to simultaneously switch the connections on the pressurized-oil supply side and output side of the pilot operating levers. This however requisitely requires not only to increase the number of parts and extend the setup space but also to inevitably complicate and size-increase the hydraulic circuits, resulting in large increase of the parts and manufacture cost hence being improper in practical application.
Accordingly, in the related-art fluid-pressure transmitting apparatus, where changing the operation pattern of the pilot operating lever, it is not easy to switch, as in the above manner, the connection over between the both pressurized-oil passages to the traveling and work equipment pilot operating levers.
Meanwhile, in the related-art fluid-pressure transmitting apparatus shown in FIG. 5, where a pressure-reducing unit for automatically changing the output pressure depending on an engine rotational speed as disclosed, e.g. in Japanese Patent Laid-Open Nos. 71353/1974 or 122363/1998 is inserted in place of the pedal-operated switching valve on the traveling pilot hydraulic circuit, if the engine speed is lowered to reduce the pressure of the pump delivery pressurized oil to be supplied to the working-machine operating device depending on the engine rotational speed, when the operating machine actuator requires a large flow amount of pressurized oil, the pressure particularly in a low engine rotational speed region is also reduced, for example, of the pilot pressurized oil outputted to an arm pilot operating valve of the arm operating device through the working-machine pilot hydraulic circuit.
Consequently, even if the pilot operating lever of the arm operating device is maximally tilt-operated, the pilot pressure outputted from the arm pilot operating valve is insufficient with a result that there occurs a case that the pressurized oil required for the working-machine actuator cannot be sufficiently supplied.
The present invention has been made in order to eliminate the above problem in the related art, and it is a concrete object thereof to provide a fluid-pressure transmitting apparatus capable of securing the operationality of a work equipment and preventing against braking and engine stall.
Furthermore, another object is to provide a fluid-pressure transmitting apparatus which makes it possible to switch over, in arrangement relationship, between the operating device for a work equipment and the operating device for traveling.
According to the present invention, a fluid-pressure transmitting apparatus having a pilot hydraulic circuit for a pump capacity control device, the fluid-pressure transmitting apparatus comprises: a pilot hydraulic circuit for driving the pump capacity control device comprising: a first operating unit connected at an input side to a first oil passage connected to a delivery passage of a pilot pump, to reduce a pressure depending on an operating amount thereof and output it to a first pilot-pressure admission passage of the pump capacity control device; a second operating unit connected at an input side to a second oil passage connected to a delivery passage of a pilot pump, to reduce a pressure depending on an operating amount thereof and output it to a second pilot-pressure admission passage of the pump capacity control device; and a low-pressure selecting valve connected at an input side to the first and second pilot-pressure admission passages, to select a lower pressure of through the first pilot-pressure admission passage and the second pilot-pressure admission passage and output the lower pressure oil to the pump capacity control device.
This invention reduces the oil pressure to be inputted to the second pilot-pressure admission passage according to an operating amount of the second operating unit, thereby reducing the capacity of the traveling pump lower than a capacity as determined by the operating amount of the first operating unit for the purpose of braking, engine stall prevention or the like.
In the fluid-pressure transmitting apparatus of this invention, the pilot hydraulic circuits for the traveling-pump capacity control device and work equipment actuator driving use pilot pumps as hydraulic sources. The pilot hydraulic circuit connected to the traveling-pump capacity control device has the first operating unit directly connected at an input side to the first oil passage branched from the delivery passage of the pilot pump, so that the delivery pressure of the pilot pump is inputted to the first operating unit to operate the first operating unit thereby being outputted as a pilot pressure to the traveling first pilot-pressure admission passage. On the other hand, the second operating unit at an input side is connected to the second oil passage connected to the delivery passage of the pilot pump so that by operating the second operating unit a pilot pressure is outputted, independently of the output from the first operating unit, to the traveling second pilot-pressure admission passage.
The pilot pressurized oils respectively outputted from the first operating unit and the second operating unit are admitted to the low-pressure selecting valve. When the pilot pressurized oil outputted from the first operating unit and second operating unit is inputted to the low-pressure selecting valve, the lower one of pilot pressurized oil is selected. The selected, lower pilot pressurized oil is admitted to the pump capacity control device of the traveling pump.
In this manner, the lower one of the pilot pressurized oils through the first and second pilot-pressure passages constituting a part of the traveling pilot hydraulic circuit is automatically selected through the low-pressure selecting valve. The low-pressure oil is admitted as a pilot pressurized oil to the capacity control device of the traveling pump. Accordingly, by operating the second operating unit, the capacity can be reduced lower than a capacity of the traveling pump as determined by the operating amount of the first operating unit at that time. As a result, the vehicle is allowed to travel at low speed or halt with greater stability than at a vehicular speed as determined by the operating amount of the first operating unit, making possible to obtain a proper brake performance or effectively realize the prevention against engine stall.
In the working vehicle of the invention, during usual travel or operation, unless the second operating unit is operated, the oil pressure outputted from the first operating unit is set lower than the oil pressure outputted from the second operating unit regardless of the first operating amount.
For example, when load is burdened on the engine, engine rotational speed decreases. However, if such decrease is abrupt, the operation of the first operating unit cannot reduce the capacity of the traveling pump to a capacity as commensurate with the engine rotational speed, thus making readily cause engine stall. Herein, if the pilot oil pressure to be inputted to the capacity control device is decrease d by the second operating unit to supply an oil pressure lower than that through the first pilot-pressure passage to the second pilot-pressure passage, the lower pilot pressurized oil is automatically selected and switched by the low-pressure selecting valve thereby preventing against engine stall.
Namely, in the state for example the first operating unit is held in a desired operating position, the second operating unit can properly control the traveling pump capacity at a capacity lower than that as determined by the operating amount of the first operating unit.
In the invention, it is preferred that an operating unit for a working machine is connected to the delivery passage of said pilot pump and, at an output side thereof, connected to said operating valve of said actuator through a driving pilot-pressure admission passage of said actuator. And an operating pattern switching valve is provided between a pilot-pressure output port of said first unit and a first pilot-pressure input port of said low-pressure selecting valve and between a pilot-pressure output port of said operating unit for actuator and a pilot-pressure input port of said operating valve for actuator, and said operating pattern switching valve communicates between the pilot-pressure output port of said first unit and the pilot-pressure input port of said operating valve for actuator.
The fluid-pressure transmitting apparatus of the invention directly connects the respective inputs of the work equipment operating unit and the first and second operating units to the pilot pump, and makes the outputs of the operating units independent to directly output the pilot pressurized oil from the work equipment operating unit to the operating valve of the work equipment actuator, thereby outputting the pilot pressurized oil from the first and second operating units through the low-pressure selecting valve to the capacity control device of the traveling pump.
According to this invention, the traveling first operating unit and the working-machine operating unit are admitted by the same pressure of pressurized oil from the same hydraulic source. The pressurized oil passed the first operating unit is admitted to one input port of the low-pressure selecting valve while the pressurized oil through the traveling second operating unit is admitted to the other input port of the low-pressure selecting valve. Consequently, in the invention, there is no need to change the arrangement of piping on the input side of the working-machine and traveling operating units (on a pressurized-oil supply side) as required in the conventional or of a two-stage switching valve for simultaneously switching the connections at the input and output of each operating unit. Without the necessity of switching the second pilot-pressure admission passage from the second operating unit, it is satisfactory to merely switch the pilot-pressure admission passages from the working-machine operating unit and traveling first operating unit in the above manner. Thus, the operating pattern switching valve can be simplified in structure, and moreover switching is easy.
Also, in the invention, it is preferred that the second operating unit is a switching valve with pedal to gradually reduce an output pressure depending on a depression amount.
In this invention, usually the switching valve with pedal is in an open state. Herein, when the pedal is depressed, the switching valve switches from a supply position toward a close position. Depending on a depression amount of the pedal, gradually decreased is the pressure of the pilot pressurized oil to be outputted from the pilot pump to the traveling second pilot-pressure admission passage. For example, even in a state the first operating unit is held, the operation of the pedal in a desired depressing position can control to reduce the capacity of the traveling pump lower than a capacity commensurate with the operation amount of the first operating unit.
Furthermore, in the invention, as a preferred form of the second operating unit, it is possible to employ a pressure-reducing valve for automatically changing an output pressure depending on an engine rotational speed, in place of the switching valve with pedal.
The output pressure of the pressure-reducing valve decreases with decrease in engine rotational speed. When the engine rotational speed decreases, reduced is the output pressure supplied from the pressure-reducing valve to the low-pressure selecting valve through the traveling second pilot-pressure admission passage. From the pressure-reducing valve, the pressurized oil at low pressure set due to the engine rotational speed is supplied to the low-pressure selecting valve. Simultaneously, if the first operating unit is operated to admit pressurized oil to the low-pressure selecting valve through the first pilot-pressure admission passage, the lower one of the pressurized oils through the first and second pilot-pressure admission passages is selected depending on an operation amount of the first operating unit. With that pressure, the capacity control device of the traveling pump is operated.
In the state that the first operating unit is fully operated and the traveling pump is in a great capacity, if a load is imposed on the engine, the engine rotational speed decreases and the output pressure of the second operating unit decreases. When the output pressure of the second operating unit becomes lower than the output pressure of the first operating unit, the pump capacity is set commensurate with the engine output thereby making possible to prevent engine stall.
Accordingly, in the traveling pilot hydraulic circuit, the lower one of the pressurized oils through the first and second pilot-pressure admission passages is always selected. Because the working-machine hydraulic circuit on one side can obtain a desired oil pressure independently of the traveling hydraulic circuit, favorable operationality is obtained even if engine rotation is varied.